1. Field of the Invention
The present invention relates to a focusing technique adopted in an imaging device equipped with an optical system including a focus lens arranged to adjust an in-focus state.
2. Description of the Related Art
The recent advancement of digital techniques has made digital video cameras and diversity of other electronic cameras popular and in widespread use. The electronic camera generally has an automatic focusing mechanism of automatically focusing a focus lens to a subject. One known technique of the automatic focusing mechanism is a hill-climbing scheme (see, for example, JP-A-2008-170507). The hill-climbing scheme of the automatic focusing mechanism shifts the position of a focus lens forward and backward along an optical axis and obtains an evaluation value (hereafter referred to as ‘AF evaluation value’) by detecting a high-frequency component of a luminance signal generated by an imaging element, such as a CCD, by means of a detection circuit. The hill-climbing scheme of the automatic focusing mechanism determines a position of the focus lens giving a peak of the AF evaluation value as an in-focus position and controls the position of the focus lens to the determined in-focus position (hereafter referred to as ‘AF control operation’). The high-frequency component of the luminance signal is used for determination of the in-focus position because of the following reason. An edge of an image taken with an electronic camera generally has a large portion of the high-frequency component. Among images of a subject taken in an identical field, the image with the greater AF evaluation value has the stronger edge or the higher level of the in-focus state.
A relation of the in-focus state according to the position of a focus lens to the AF evaluation value is shown in FIG. 5. As shown in FIG. 5, the AF evaluation value reaches its maximum at the in-focus position and decreases with an increase of the distance away from the in-focus position. The AF evaluation value has no further decrease at positions of the focus lens away from the in-focus position by a preset distance. In a significantly defocused state that is incapable of determining the contents of an image, there is no variation in frequency of the luminance signal. In this state, there is no variation in AF evaluation value. The AF evaluation value with no further variation is called an offset evaluation value AFoff.
The AF control operation is described with reference to FIG. 6. The AF control operation first identifies a moving direction of the focus lens that increases the AF evaluation value by a wobbling technique. The wobbling technique minutely shifts the position of the focus lens and detects a variation of the AF evaluation value in the course of the minute positional shift, so as to identify the moving direction of the focus lens that increases the AF evaluation value to a peak. As shown in FIG. 6, the AF control operation by the wobbling technique starts at a position P1 of the focus lens. A direction going from the position P1 toward a peak (toward a position P3) is identified as the moving direction of the focus lens. The AF control operation then shifts the position of the focus lens by every predetermined distance in the identified moving direction and detects the AF evaluation value at every shifted position. When the AF evaluation value detected at a certain shifted position is greater than the AF evaluation value previously detected, the AF control operation updates a maximum value AFmax of the AF evaluation value.
When the AF evaluation value reaches the peak at the position P3 and then decreases to a threshold value TH, that is, when the focus lens moves to a position P2, the AF control operation determines the position P3 of the focus lens that gives the maximum value AFmax of the AF evaluation value as an in-focus position and moves the focus lens to the determined position P3. The peak position or the position of the focus lens giving a peak of the AF evaluation value is determined only after the AF evaluation value decreases from the maximum value AFmax to the threshold value TH. The video signal generally includes random noise varying with time. This random noise causes a certain variation of the AF evaluation value even when the focus lens is kept at a fixed position. The AF control operation takes into account a potential error induced by such a variation to determine the peak position with high accuracy. The threshold value TH is generally set to a result of subtraction of a preset fixed rate from the maximum value AFmax.
The automatic focusing mechanism, however, has some problems arising in certain imaging conditions. In the case of taking an image of a subject with high contrast, a luminance signal generated by the imaging element includes a large portion of the high-frequency component. A relatively large AF evaluation value is thus detected by the automatic focusing mechanism. In the case of taking an image of a subject with low contrast, on the other hand, a luminance signal generated by the imaging element includes a small portion of the high-frequency component. A relatively small AF evaluation value is thus detected by the automatic focusing mechanism. A high-speed positional shift of the focus lens leads to a greater integral effect of the optical system. This significantly decreases the high-frequency component and thereby gives a relatively small AF evaluation value. A low-speed positional shift of the focus lens, on the other hand, leads to a smaller integral effect of the optical system. This does not significantly decrease the high-frequency component and thereby gives a relatively large AF evaluation value.
Concrete examples of such phenomena are shown in FIGS. 7A and 7B. FIG. 7A shows variations of the AF evaluation value at various moving speeds of the focus lens with respect to a high-contrast subject. FIG. 7B shows variations of the AF evaluation value at the various moving speeds of the focus lens with respect to a low-speed subject. As clearly understood from the comparison between FIG. 7A and FIG. 7B, the low-contrast subject has smaller peak values of the AF evaluation value than the high-contrast subject, because of the difference in integral effect of the optical system explained above. In both of the high-contrast subject and the low-contrast subject, the peak value of the AF evaluation value decreases in the sequence of a low moving speed, a medium moving speed, and a high moving speed of the focus lens.
As shown in FIG. 7A, the high-contrast subject gives relatively large AF evaluation values. Even in the condition of the high moving speed of the focus lens, the threshold value TH calculated by multiplying a maximum value AFmax by a preset rate R (0<R<1) is greater than the offset evaluation value AFoff. The peak position is thus always determinable. As shown in FIG. 7B, on the other hand, the low-contrast subject gives relatively small AF evaluation values. In the condition of the high moving speed of the focus lens, the threshold value TH calculated by multiplying a maximum value AFmax by the preset rate R may be smaller than the offset evaluation value AFoff. The peak position may thus be not determinable.
One possible measure to solve the problem may set a large value to the threshold value TH. Setting a large value to the threshold value TH, however, can not sufficiently eliminate the potential effect of noise especially in the state of the low-speed positional shift of the focus lens. This undesirably increases the potential for erroneous determination of the in-focus position. Setting a small value to the threshold value TH lowers the potential for erroneous determination of the in-focus position but increases the moving amount of the focus lens by the AF control operation. This causes a higher vibration degree of the imaging result, which makes the user feel uncomfortable. In general, the threshold value TH is set by taking into account the balance between the accuracy of determination of the in-focus position and the vibration degree of the imaging result. Another possible measure to solve the problem may set a low value to the moving speed of the focus lens with respect to the low-contrast subject. Setting the low value to the moving speed, however, undesirably extends the time required for the AF control operation.